1. Field of the Invention
The present invention generally relates to a laser driving technique used in an image reproducing apparatus, and more particularly, to a laser modulating and driving device that produces a pair of small swing differential modulation signals to drive a laser diode (LD) used as a light source in an image reproducing apparatus.
2. Description of Related Art
In general, a semiconductor laser (LD) or a semiconductor laser array (LD array) is used as a light source of image reproducing apparatuses, including laser printers, optical recorder/players, digital photocopying machines, and optical communication apparatuses.
There are several methods for modulating the optical output of a semiconductor laser. For instance, power modulation for modulating the light quantity itself, pulse width modulation for modulating the light emitting time, and a combination of the power modulation and the pulse width modulation are known. An example of pulse width modulation is generating a triangular wave or a sawtooth wave corresponding to a pulse period and comparing the generated wave to an analog video signal to produce a pulse-width modulated signal. Another example of pulse width modulation is generating a high-frequency clock, dividing the clock in a digital manner to produce a delayed pulse, and producing a pulse-width modulated signal by the logical sum or logical multiplication.
FIG. 1 illustrates a general structure of a conventional laser modulating and driving device used in a laser printer or other image reproducing apparatuses. The pixel data generating unit 1110 converts input data to pixel data suitable to image-forming factors, including the γ characteristic of the photosensitive drum. The LD modulation signal generating unit 1120 produces a modulation signal for power-modulating or pulse-width-modulating the optical output of the semiconductor laser 190 according to the pixel data. The LD driving unit 1170 drives the semiconductor laser 190 based on the modulation signal.
Conventionally, the LD modulation signal generating unit 1120 and the LD driving unit 1170 are arranged in a single block 1020. These elements are mounted on the same printed circuit board (PCB) or fabricated as the same integrated circuit (such as ASIC), and the block 1020 is separated from a block 1010 of the pixel data generating unit 1110. This is because the transmission rate of the modulation signal is higher than that of the pixel data, and therefore it is desired to arrange the LD modulation signal generating unit 1120 and the LD driving unit 1170 as close to each other as possible.
Japanese Patent No. 3283256 discloses an image forming apparatus proposed from the same viewpoint. In the publication, a PWM circuit corresponding to the LD modulation signal generating unit 1120 and a laser driver corresponding to the LD driving unit 1170 are mounted on the same circuit board, while the digital tone control circuit corresponding to the pixel data generating unit 1110 is mounted on a separate circuit board. Data and clocks are supplied from the digital tone control circuit to the PWM circuit by differential transmission.
Japanese Patent Laid-open publication No. 11-105336 discloses a semiconductor integrated circuit which is also proposed from the same viewpoint. In the publication, a pixel modulation circuit implemented as the PWM circuit, a laser driving circuit, and a connection circuit between them are mounted on a single chip. Signals transmitted between the pixel modulation circuit and the connection circuit, as well as signals transmitted between the connection circuit and the laser driving circuit, are differential signals.
However, demand for high-speed operation in image forming and/or reproducing apparatuses is growing more and more, and the number of lasers is increasing. To form a single color image, two, four, or eight (depending on the case) lasers, which may be arranged as an LD array, are used. In addition, photocopiers and printers have shifted from monochrome to color, and multiple LD modulation signal generating units and the corresponding number of LD driving units are required.
For example, in order to form a 4-color image using two semiconductor lasers based on 8-bit pixel data and a 1-bit LD modulation signal (pulse), then 64-bit pixel data and eight modulation signals are required. Using the conventional laser modulating and driving device shown in FIG. 1, 64-bit digital pixel signals have to be transmitted from the pixel data generating unit 1110 to the LD modulation signal generating unit 1120 at a high rate, although not required as fast as transmission of LD modulation signal. The number of data lines increases and the size and the structure of the data transmission part become large and complicated, which makes it difficult to realize high-speed transmission of 64 data signals. If such digital pixel signals are supplied from the pixel data generating unit 1110 to the LD modulation signal generating unit 1120 by differential transmission, the structure of the data transmission part and the transmission path becomes larger and more complicated.
Another problem is inconsistency in conditions of a supply voltage for laser driving and a supply voltage for generation of modulation signals. Along with the demand for high-speed operation, integralization of a device for generating modulation signal with other devices on a chip is advancing, and the supply voltage of such devices on the chip is becoming lower and lower. In contrast, the wavelength of the semiconductor laser becomes shorter and shorter for achieving high resolution, and the falling voltage (i.e., the potential drop between terminals) of the semiconductor laser tends to increase, causing the supply voltage for driving the semiconductor laser to be increased. Thus, inconsistency between the power supply voltage condition for laser driving and that for modulation signal generation becomes conspicuous.